This invention is related to a method for forming carbonaceous preforms, e.g., that can be infiltrated with molten silicon to form silicon carbide bodies or composites. More specifically, it is a method for forming a water based slurry mixture of carbonaceous material into a porous preform.
Infiltration of porous carbonaceous preforms with molten silicon or silicon alloy to form silicon carbide composites is disclosed, for example, in U.S. Pat. Nos. 4,944,904, 5,021,367, 5,043,303, 4,889,686, and 4,981,822, incorporated herein by reference. The preform is formed from a carbonaceous material at least comprised of carbon powder or fibers. The carbonaceous material may also include powders of a metal which reacts with molten silicon to form a metal silicide, such as chromium, molybdenum, tantalum, titanium, tungsten, and zirconium; or a ceramic material which may or may not react with molten silicon including carbides of boron, molybdenum, niobium, silicon, and titanium; nitrides of aluminum, niobium, silicon, titanium, and zirconium; and silicides of chromium, molybdenum, tantalum, titanium, tungsten, and zirconium.
A method of forming the porous carbonaceous preforms from carbon fibers is disclosed in U.S. Pat. No. 4,320,079, incorporated herein by reference. Carbon fiber and a curable binder are blended to produce a stable pourable suspension. The suspension is degassed by use of reduced pressure, and poured into a mold. Excess binder is stripped from the mold, and residual binder is cured to produce a self-supporting shaped preform. Suitable binders for use in the method included, epoxy resins, Krylon, alginates, guargum, and white glue.
While carbon has a low density, other carbonaceous materials such as the reactive metals can have a higher density, and the binder should maintain a homogeneous distribution of the components so that the higher density components do not separate or settle when the carbonaceous material is cast to form the preform. The binder for the carbonaceous material should decompose to leave no residue that is undesirable for the later infiltration of the preform.
Silicon carbide bodies and silicon carbide matrix composites have been formed by the molten silicon infiltration of porous carbonaceous preforms. During infiltration of the porous carbonaceous preforms, there is a reaction between the molten infiltrant and carbon or reactive metal in the preform to form silicon carbide or metal silicides. The reaction closes infiltration pore channels in the preform in direct competition with the continued infiltration of the preform, and is known as reaction-choking. A homogeneous distribution of open porosity in the preform provides a continuous infiltration path, and penetration of the infiltrant throughout the preform.
Low density amorphous carbon fibers or carbon particles having a density of about 1.2 grams per milliliter, increase in volume by about 25 percent when reacted with molten silicon to form silicon carbide. Graphitic carbon, having a higher density of about 2.2 grams per milliliter, increases in volume about 128 percent when reacted with molten silicon to form silicon carbide. Reaction of molten silicon with molybdenum or molybdenum carbide produces volumetric increases of about 159 percent and 176 percent, respectively, in converting to the solids molybdenum disilicide, or molybdenum disilicide plus silicon carbide. When the volume of silicon consumed in such reactions is included, then there is a net volumetric decrease of about 25 percent. Uniform distribution of the carbon powder, fiber, reactive metal, ceramic material, and porosity in the preform minimizes inhomogeneous volumetric changes that occur during infiltration to minimize preform cracking, choking-off of the infiltrant, and retention of latent stresses in the reaction formed body.
One aspect of this invention is to provide a method of forming the carbonaceous material in a water-based slurry mixture into the porous preform.
Another aspect of this invention is to provide a method of forming the porous carbonaceous preform with a uniform distribution of the carbonaceous material and porosity.